It is well known that in perforating earthen formations to tap subterranean deposits such as gas or oil, that perforation is accomplished by well drilling tools and a drilling fluid. The drilling fluid serves to cool and lubricate the drill bits, to carry the cuttings to the surface as the drilling fluid is circulated in and out of the well, to support at least part of the weight of the drill pipe and drill bit, to provide a hydrostatic pressure head to prevent caving of the walls of the well bore, to deposit on the surface of the well bore a filter cake which acts as a thin, semi-impervious layer to prevent undue passage therethrough of fluids, and to perform other functions as are well known in the drilling art. It is important that the drilling fluid exhibit a relatively low rate of filtration or fluid loss in addition to having desirable rheological properties such as viscosity and gel strength. It is also important that the drilling fluid system should be kept as simple and inexpensive as possible in order to avoid undue expense in the drilling of the well.
Drilling fluid also has an effect on the friction between the drill pipe and the bore hole, and the higher the coefficient of friction between the drill pipe and the formation being drilled, that is, the lower the degree of lubricity of the drilling fluid, the greater the power requirements needed to rotate the drill pipe in the bore hole filled with the drilling fluid. Further in this respect, a drilling fluid having a low degree of lubricity or a high coefficient of friction between the drill pipe and the uncased well bore means that a high degree of drag on the drill pipe results, thereby lessening the useful life of the drill pipe. Therefore, the lubricating properties of the drilling fluid are assuming an increased importance to those skilled in the art, not only with regard to the wearing of the bearings in the drill bit, but also with respect to the friction between the drill pipe and the uncased bore hole.
Drilling an oil or gas well is generally conducted by a rotary system. This system depends upon the rotation of a string of drill pipe to the bottom of which is attached a multi-pronged drilling bit. The bit cuts into the earth causing the cuttings to accumulate as drilling continues. As a result, a drilling fluid must be used to carry these cuttings to the surface for removal, thus allowing the bit to continue functioning and the bottom hole to be kept clean and free of cuttings at all times. Drilling systems other than the rotary system are sometimes used, but these also require a drilling fluid to remove the bore hole cuttings and to perform functions related to drilling fluids.
Oil-producing formations are generally porous layers having varying degrees of permeability to the flow of fluids such as oil, water or gas. Consequently, the rate of oil production is largely determined by the rate of flow through these permeable formations which, in turn, is dependent upon the porosity or permeability of the sand or stone present. In drilling through such a porous layer, it is desirable to employ a drilling fluid having such characteristics that excessive amounts of liquids or solids are prevented from penetrating through the porous formation. The ability of the drilling fluid to prevent excessive formation fluid penetration is called filtration control.
Besides the filtration control, an acceptable polymer has to maintain an adequate rheology (flow) properties of drilling fluids. Gary and Darley, ed., Composition and Properties of Oil Well Drilling Fluids, 4th ed., page 12, states: "The flow properties of the drilling fluid play a vital role in the success of the drilling operation. These properties are primarily responsible for removal of the drill cuttings, but influence drilling progress in many other ways. Unsatisfactory performance can lead to such serious problems as bridging the hole, filling the bottom of the hole with drill cuttings, reduced penetration rate, hole enlargement, stuck pipe, loss of circulation, and even a blowout."
Materials that have been used in the past to control filtration rates of aqueous drilling fluids by plugging, producing cakes or similar methods, have included materials such as pregelatinized starch, sodium carboxylmethylcellulose (CMC), sodium polyacrylates and lignites. Each of these materials have certain limitations. For example, lignite becomes ineffective in high salt concentrations. Thermodegradation of CMC is accelerated as the temperature approaches 300.degree. F.
Acrylic and methacrylic derivatives, such as those which are copolymerized with hydrocarbon substituted styrenes, such as alpha methyl styrene, para methyl styrene, 2,4-dimethyl styrene and the like have been utilized in drilling fluids. For example, U.S. Pat. No. 2,718,497 teaches the use of relatively high molecular weight polymers of these materials to control water loss characteristics of aqueous muds and clay dispersions. Additionally, U.S. Pat. No. 2,650,905 teaches the use of water soluble sulfonated polystyrene derivatives for filtration control in water-based drilling fluids.
Acrylic acid derivatives such as copolymers of acrylamide and sodium acrylate derivatives have been frequently used commercially as flocculants for drilling fluids, and are disclosed in U.S. Pat. Nos. 3,558,545 and 3,472,325. Similarly, a copolymer derived from acrylic acid and acrylamide is disclosed in U.S. Pat. No. 3,323,603 as a flocculant for aqueous drilling fluids. However, the use of polyacrylate for filtration control in some areas has been limited by its sensitivity to calcium ions.
U.S. Pat. No. 4,293,427 discloses a copolymer additive prepared from (1) a (meth)acrylamido alkyl sulfonic acid or alkali metal salt thereof and (2) a (meth)acrylamide or N-alkyl (meth)-acrylamide. The copolymer may be cross-linked with a quaternary ammonium salt.